The present invention relates to microwave pulse generators capable of providing very high power pulses. This type of generator is applicable especially in the field of radar and scientific or industrial instruments such as electron accelerators, for example.
In these fields, it is sometimes sought to obtain narrow pulses of a few nanoseconds with a repetition frequency of one or more hundred Hertz. The peak output powers are of the order of a megawatt up to a hundred or so megawatts in the S band or in the X band. Of course, it is also sought to reduce the size of the generator as much as possible so that its diameter does not exceed about 10 centimeters and its length 1 meter to 1.5 meters.
Reference may be made to FIG. 1a. The microwave pulse generators which come closest to providing the required electrical performance comprise a magnetron 1 generating pulses and this magnetron 1 cooperates with a resonant compression cavity 2 in order to compress the pulse delivered so as to reach the desired peak power and pulse width. While the magnetron 1 emits a pulse of power P1 and of length t1, the resonant compression cavity 2 is closed, thereafter, it is open so as to be emptied quickly in a time t2 which is less than t1, into a user device 9, for example an emitting antenna shown diagrammatically by a horn. The closing and opening take place in a region of the cavity with the reference 6. At the output of the resonant compression cavity 2, except for losses, a pulse of power P2 is obtained such that P2.t2≅P1.t1, since there is conservation of energy.
FIG. 1b illustrates schematically the appearance of the pulse stored during the time t1 and that of the pulse released during the time t2.
The resonant cavity 2 can be opened and closed by means of an electric-arc switching device 3 possibly with an ionizable gas. The exemplary switching device 3 shown in FIG. 1a comprises an extension filled with an ionizable gas which is coupled to the resonant compression cavity 2, this extension has a bottom 4 acting as short circuit. Away from the bottom 4, a microwave window 8 separates the extension from the resonant compression cavity 2. The bottom 4 is separated, by a multiple h of a half wavelength xcex/2 of the wave stored in the resonant compression cavity 2, from the region 6 of the resonant compression cavity 2. In the region 6, it restores a short circuit 5 which closes the resonant compression cavity 2. This extension comprises a device 7 which creates an electric arc, therefore a new short circuit 12, separated, by an odd multiple (2k+1) of a quarter wavelength xcex/4, from the bottom 4. This short circuit 12 restores an open circuit, which opens the cavity into the user device 9, to the region 6.
The switching frequency corresponds to the desired repetition frequency and it is synchronized with a falling edge of the pulse to be compressed.
This type of pulse generator has the advantage of being simple and relatively compact, but has the major drawback of operating very poorly or not at all if the frequency of the magnetron 1 is not equal to the resonant frequency of the resonant compression cavity 2. This is because the resonant compression cavity 2 has a very high Q factor, for example about 10 000, in order to be able to store as much energy as possible. This value imposes, for example at 10 GHz, an accuracy of better than plus or minus 0.5 MHz on the frequency of the magnetron. Now, after transport and a dismantling-reassembly operation, during temperature variations, it is not unusual for the frequency of the magnetron to be offset by more than a megahertz.
The conventional solution is then to add a system 11 which searches for the resonant frequency of the resonant cavity and to automatically control the tuning system 10, which is generally motor operated, and which tunes the frequency of the magnetron 1 to this resonant frequency. However, the pulse generator then becomes heavy, larger and loses some of its simplicity. Also, if there is no search system, before each use, an adjustment has to be carried out which takes time and which is not compatible with the use that it is desired to make thereof under many circumstances.
The aim of the present invention is to overcome these frequency-adjustment and size problems.
In order to achieve this, the present invention proposes a pulse generator comprising a microwave source which delivers a pulsed signal into a resonant compression cavity equipped with a switching device for opening it or closing it, this pulsed signal being stored in the resonant compression cavity when closed in order to be compressed therein before being delivered with a greater amplitude and a smaller width when the cavity is open, characterized in that the microwave source is of the amplifier type with an instantaneous bandwidth including the resonant frequency of the resonant compression cavity, and in that it is fed an input signal to be amplified which is taken off from the resonant compression cavity.
With a structure of this sort, the frequency of the signal delivered by the microwave source is fully equal to the resonant frequency of the resonant compression cavity.
A solid-state amplifier or a longitudinal interaction microwave amplifier tube can be used as an amplifier.
The switching device may comprise a wall forming a short circuit which restores a short circuit in the resonant compression cavity in order to close it and means to strike an electric arc which restores an open circuit in place of the short circuit in order to open it.
The wall forming a short circuit may be the bottom of a waveguide portion connected to the resonant compression cavity.
The electric arc may be set up in the waveguide portion or else in the resonant compression cavity, which has the effect of changing its resonant frequency and of removing the closing short circuit.
The switching device may operate with an ionizable gas which facilitates the ignition of the electric arc.
The electric arc may be set up at bosses or points which have the effect of reinforcing the electric field wherever they are. It may also be set up in a small pressurized quartz tube which channels and optimizes the discharge.
The electric arc may be struck by radiation or may strike automatically when the electric field is sufficiently high.
When the microwave source is a microwave tube, in order to save on radial size, the resonant compression cavity may be made by its electron collector.
In this configuration, the collector is separated from the output cavity by a wall comprising a through-orifice for the electrons and a device for coupling the cavity to the collector which passes through the wall and which is distinct from the through-orifice for the electrons.
For the purpose of limiting the length of the collector, it is possible to equip the latter with deflection means in order to deflect the electrons in a suitable manner. The length traveled by the pulsed signal delivered by the microwave source up to the resonant compression cavity, in the resonant compression cavity then up to the microwave source and in the microwave source preferably introduces a phase shift which is a multiple of 2xcfx80 at the resonant frequency of the compression cavity, so enabling the amplifier to be transformed into an oscillator.
Other features and advantages of the invention will become apparent on reading the following description illustrated by the appended figures which show: